Reference collections Mass spectrometers

Both the GC-MS and GC-IR instruments obviously require that the column effluent be fed into the spectrometer detection path. For the IR instrument, this means that the IR cell, often referred to as a light pipe, be situated just outside the interferometer (Chapter 8) in the path of the light, of course, but it must also have a connection to the GC column and an exit tube where the sample may possibly be collected. The infrared detector is nondestructive. With the mass spectrometer detector, we have the problem of the low pressure of the mass spectrometry unit coupled with the ambient pressure of the GC column outlet. A special method is used to eliminate carrier gas while retaining sufficient amounts of the mixture components so that they are measurable with the mass spectrometer. 

The mass spectrometer when used as a detector for liquid chromatography is a universal detector and in most instances has no influence on the chromatographic separation. LC/MS spectra obtained from both volatile and low- volatile compounds are similar to ordinary El spectra and may be interpreted by comparison with normal reference spectra collections. Interpretable mass spectra can also be obtained from nonvolatile compounds. 

Tor reference. Positive identification can be made only by collecting the compound or transierring it as it elutes directly into another apparatus for analysis by other means, such as infrared or ultraviolet spectroscopy, mass spectrometry, or nuclear magnetic resonance. Commercially available apparatus is available which combines in a single unit both a gas chromatograph and an infrared, ultraviolet, or mass spectrometer for routine separation and identilicalion. The ancillary system may also be microprocessor-based, with an extensive memory for storing libraries of known infrared spectra or fragmentation patterns (in the case of mass spectrometers). Such systems allow microprocessor-controlled comparison and identilicalion of detected compounds. 

Identification of the volatile constituents was performed by GC-MS. After separation (under similar conditions as those described above) identification was carried out using a Nermag R10-10 mass spectrometer (Rueil-Malmaison, France). Mass spectra were prepared at an ionizing voltage of 70 eV and compared with those included in the NBS library system. The Institute s own collection of reference spectra were also used. 

All major mass spectral data collections consist of El mass spectra, mostly recorded under accepted standardized conditions such as an ionization voltage of 70 eV, an emission current of 100-200 xA, and an ion source temperature of 150-200°C. Several types of GC/MS systems may be applied, for instance, magnetic sector, quadrupole, or ion trap analyzers. Ion trap systems are considered less applicable, when data comparison is required with spectra from a reference library. In particular, basic compounds related to VX or the three nitrogen mustards tend to produce protonated molecular ions by self-protonation. Magnetic sector and quadrupole mass spectrometers suffer less from interference of self-protonation, and spectra produced with these types of instruments are generally reproducible. 

In 2005, De Laeter discussed the role of isotope reference materials for the analysis of non-traditionaT stable isotopes. At present, no isotopically certified reference materials exist for a large number of elements, including Cu, Zn, Mo and Cd, and it is important that this situation be rectified as soon as practicable. Before the isotopically certified reference materials become available for selected elements, suitable reference materials can be created as a standard if sufficient and reliable isotope data have been obtained by interlaboratory comparisons. For example, the Hf/ Hf isotope ratio was measured using hafnium oxide from Johnson Matthey Chemicals, JMC-475, for hafnium isotope ratio measurements with different multi-collector mass spectrometers (ICP-MS and TIMS) as summarized in Table 8.1. However, no isotope SRM is certified for the element Mo either. Mo isotope analysis is relevant, for example, for studying the isotope fractionation of molybdenum during chemical processes or the isotope variation of molybdenum in nature as the result of the predicted double (3 decay of Zr or 18.26-28 spectroscopically pure sample from Johnson Mattey Specpure is proposed as a laboratory standard reference material if sufficient and reliable isotope data are collected via an interlaboratory comparison. 

When only a few analytes are of interest for quantitative analysis and their mass spectrum is known, the mass spectrometer can be programmed to monitor only those ions of interest. This selective detection technique is known as selected ion monitoring (SIM). Because SIM focuses on a limited number of ions, more signal can be collected for each selected mass. This increases the signal-to-noise ratio of the analyte and improves the lower limit of detection. In general, however, an unknown is considered identified if the relative abundances of three or four ions agree within 20% of those from a reference compound. 

While MALDI is a typical off-line ionization technique/interface, one can also implement classical interfaces to carry out off-line (or at-line) analyses of reaction mixtures. Aliquots can be obtained from a reaction mixture (a dynamic sample/matrix) at specific time points, and injected to the ion source of a mass spectrometer for analysis (e.g., [49-54]). In some cases, quenching is conducted [49, 50]. If the transient intermediates (e.g., radicals) are to be detected, it is important to assure that the reaction has not finished at the time of ionization [55]. Short-lived intermediates can be reacted with auxiliary compounds in order to enable subsequent measurements in the methodology referred to as spin-trapping (see, e.g., [56, 57]). The off-line analyses based on aliquoting of dynamic matrices, and subsequent separation, provide temporal resolutions in the order of a few minutes (see, e.g., [58]). Nonetheless, they are uncomplicated, and - in some cases - they may enable offline sample treatment prior to detection by MS. For instance, inorganic ions, present in the reaction mixture, can readily be removed on an exchange resin to render the collected samples compatible with MS [59]. 

SIMS derives compositional information by directing a focused energetic ion beam at the surface of interest. These ions, referred to as primary ions, induce the emission of atoms and molecules from the solid s surface, a small percentage of which exist in the ionized state. The emitted ions, referred to as secondary ions, are then collected and passed through a mass spectrometer, hence the name secondary ion mass spectrometry. 

Two new ionization methods to appear recently are desorption ESI (DESI) and direct analysis in real time (DART). They are the first in a new area of ionization mechanisms that are collectively referred to as open-air ionization. Both DART and DESI form ions in an open atmosphere and sample the resulting plume through an entrance cone into the mass spectrometer. 

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